This invention relates to air separation.
Modern industry and chemical processes including an oxidation step call for ever larger quantities of oxygen in order to perform that step. Oxygen can be produced in quantities in excess of 2000 tons per day by an air separation process which comprises compressing an air stream, purifying the air stream by removing therefrom components of relatively low volatility such as water vapour and carbon dioxide, cooling the thus purified air stream to a temperature suitable for its separation by fractional distillation or rectification, and then performing that separation so as to produce an oxygen product of desired purity. The purification is preferably performed by using beds of adsorbent which adsorb the components of low volatility such as water vapour and carbon dioxide. The fractionation of the air is preferably performed in a double column comprising a higher pressure column and a lower pressure column which share a heat exchanger that condenses nitrogen at the top of the higher pressure column and reboils oxygen-rich liquid at the bottom of the lower pressure column. Some of the thus formed liquid nitrogen is used as reflux in the higher pressure column while the remainder is removed from the higher pressure column, is sub-cooled, and is passed through an expansion valve into the top of the lower pressure column so as to provide reflux for that column. The air is introduced into the high pressure column. Oxygen-enriched liquid air is withdrawn from the bottom of the high pressure column and is passed to the lower pressure column where it is typically separated into substantially pure oxygen and nitrogen products. These products may be withdrawn from the lower pressure column in the gaseous state and warmed to ambient temperature in countercurrent heat exchange with the incoming air, thereby effecting the cooling of the incoming air. Since the process operates at cryogenic temperatures, refrigeration has to be generated. This is typically done either by expanding a part of the incoming air in a turbine or by taking a stream of nitrogen from the high pressure column and passing it through an expansion turbine.
Such air separation plants are nowadays very common. Almost universally, the lower pressure column operates at 1.3 to 1.7 bar and the higher pressure column at pressures in the range 5.5 to 6.5 bar. The reason for choosing these operating pressures is that it enables product nitrogen and oxygen streams, after being warmed to ambient temperature, to be at a pressure a little above atmospheric.
In practice, mechanical engineering and transport constraints place an upper limit on the size of such an air separation plant when the columns are to be fabricated remote from the intended site of the air separation plant. Expressed in terms of the tonnage of oxygen produced per day from the plant, this limit is in the order of 2,500 tons per day. Thus, the so-called Sasol process for the production of oil from coal, since it has a demand for oxygen well in excess of 5,000 tons per day, uses several separate air separation plants to fulfil the oxygen demand.
It has been proposed to operate the higher pressure and lower pressure columns at pressures substantially in excess of the conventional ranges of 5.5 to 6.5 and 1.3 to 1.7 bar respectively. The prime reason for using such higher pressures is to obtain more efficient separation in the low pressure column. A disadvantage of such proposals is that when there is insufficient demand for all the nitrogen that is produced, there is a problem of what to do with the resulting higher pressure product nitrogen stream. It has been proposed to solve this problem by recovering energy from the nitrogen stream by expanding it in a turbine and to use this energy in generating electricity for export. Such proposals are indeed generally advantageous. There are some locations, however, where the export of electricity is not possible or desirable. The invention relates to an alternative method and apparatus for taking advantage of the nitrogen product.